Radiative forcing

Radiative forcing (or climate forcing) is a concept used in climate science to quantify the change in energy balance in the Earth's atmosphere caused by various factors, such as concentrations of greenhouse gases, aerosols, and changes in solar radiation. In more technical terms, it is "the change in the net, downward minus upward, radiative flux (expressed in W/m²) due to a change in an external driver of climate change."^: 2245 These external drivers are distinguished from feedbacks and variability that are internal to the climate system, and that further influence the direction and magnitude of imbalance.

A planet in radiative equilibrium with its parent star and the rest of space can be characterized by net zero radiative forcing and by a planetary equilibrium temperature.

Radiative forcing is not a "thing" in the sense that a single instrument has independently measured it. Rather it is a useful scientific concept whose strength can be estimated from more fundamental physics principles. Radiative forcing can also be calculated based upon observation and analysis of changes in other measurable atmospheric parameters.^: 1–4

Radiative forcing on Earth is meaningfully evaluated at the tropopause and at the top of the stratosphere. It is quantified in units of watts per square meter, and often summarized as an average over the total surface area of the globe. Radiative forcing varies with solar insolation, surface albedo, and the atmospheric concentrations of radiatively active gases – commonly known as greenhouse gases – and aerosols.

The IPCC summarised the current scientific consensus about radiative forcing changes as follows: "Human-caused radiative forcing of 2.72 W/m² in 2019 relative to 1750 has warmed the climate system. This warming is mainly due to increased GHG concentrations, partly reduced by cooling due to increased aerosol concentrations".^: 11

The atmospheric burden of greenhouse gases due to human activity has grown especially rapidly during the last several decades (since about year 1950). For carbon dioxide, the 50% increase (C/C₀ = 1.5) realized as of year 2020 since 1750 corresponds to a cumulative radiative forcing change (ΔF) of +2.17 W/m². Assuming no change in the emissions growth path, a doubling of concentrations (C/C₀ = 2) within the next several decades would correspond to a cumulative radiative forcing change (ΔF) of +3.71 W/m².

Radiative forcing can be a useful way to compare the growing warming influence of different anthropogenic greenhouse gases over time. The radiative forcing of long-lived and well-mixed greenhouse gases have been increasing in earth's atmosphere since the industrial revolution. CO₂ dominates the total forcing, with methane and chlorofluorocarbons (CFC) becoming relatively smaller contributors to the total forcing over time. The five major greenhouse gases account for about 96% of the direct radiative forcing by long-lived greenhouse gas increases since 1750. The remaining 4% is contributed by the 15 minor halogenated gases.